Patent Application
20080075842
The novel features and advantages of the present invention will best be understood by reference to the detailed description of the preferred embodiments which follows, when read in conjunction with the accompanying drawings, in which:
The various features of the preferred embodiment(s) will now be described with reference to the drawing figures, in which like parts are identified with the same reference characters. The following description of the presently contemplated preferred embodiments of practicing the invention are not to be taken in a limiting sense, but are provided merely for the purpose of describing the general principles of the invention.
Introduction
The present invention comprises a system and method for manufacturing membrane electrode assemblies (MEA's). MEA's are used in fuel cells, and in this particular application, in direct methanol fuel cells (DMFC's). An MEA comprises a catalyst coated membrane (CCM) disposed between two diffusion layers. A CCM comprises an electrolytic membrane, in a preferred embodiment, a sulfonated tetrafluoroethylene copolymer such as NAFION®, having opposing major planar surfaces, an anode catalyst layer disposed on a first major planar surface, and a cathode catalyst layer disposed on a second major planar surface (catalyst layers may be applied in either order). According to several aspects of the present invention, the catalyst layers are formed on the major planar surfaces of the electrolytic membrane, preferably through a spraying process. The catalyst layers are preferably substantially porous and electrically conductive. The diffusion layers also preferably are electrically conductive and allow for the flow of reactants toward the catalyst layers and the removal of reaction products away from the catalyst layers.
How the catalyst layers are formed on the electrolytic membrane may substantially impact the efficiency and usefulness of the DMFC. According to an embodiment of the present invention, either or both catalyst layers are formed from a sprayable catalyst ink comprising catalyst aggregates, preferably having an average size of from about 0.5 μm to about 20 μm, e.g., from about 0.7 μm to about 15 μm or from about 1 μm to about 10 μm, a liquid vehicle and optionally electrolytic particles (e.g., NAFION® particles). According to a preferred embodiment of the present invention, the liquid vehicle comprises, consists essential of, or consists of water. The sprayable ink preferably is dispelled from a spraying mechanism (e.g., “sprayed”), to form an aerosol comprising a plurality of droplets, the droplets preferably having an average droplet size of from about 10 μm to about 100 μm, e.g., from about 20 μm to about 70 μm or from about 30 μm to about 50 μm, preferably about 40 μm. Before, during and/or after droplet formation, substantially spherical agglomerates of catalyst and, if present, electrolytic particles, e.g., NAFION®, are formed. As the agglomerates hit the membrane surface and are heated, they mesh with each other, forming a catalyst layer (which optionally includes NAFION®) that desirably is substantially porous. The steps of spraying and heating can be repeated numerous times, to create catalyst layers of any desired thickness. The substantially spherical agglomerates cause the catalyst layer to be substantially porous, thereby improving the efficiency of the electrochemical reactions occurring at the interface between the cathode catalyst layer and the membrane and the interface between the anode catalyst layer and the membrane.
Porosity, as is understood by those of ordinary skill in the art of the present invention, describes how densely a certain material is packed. Porosity can be defined by the amount of non-solid volume to the total volume of a material, although as one of ordinary skill in the art knows, other definitions exist. Porosity (Φ) can be defined, for example, by the following ratio:
wherein Vp is the non-solid volume (pores and liquid) and Vm is the total volume of material, including the solid and non-solid parts. According to this ratio, the porosity value is a fraction, between 0 and 1, with porosity increasing as the value approaches 1. According to an exemplary embodiment of the present invention, the catalyst layer has a porosity in the range of from about 0.20 to about 0.60, e.g., from about 0.30 to about 0.55 or from about 0.40 to about 0.50.
In a preferred aspect of the invention, prior to any steps of spraying, heating, or as described below, pre- and post-conditioning, the membrane is applied to a frame. The frame has several unique characteristics that assist in manufacturing membrane electrode assemblies of nearly any desired shape, optionally autonomously, and with little effort (e.g., manually or via fully automated software and computer controlled machines). Frames are preferably rigid and comprise alignment structures. The alignment structures can be one or more alignment pins or alignment holes (e.g., for receiving alignment pins), of any size or shape, or can be an optically reflective or transmittable material or device. The alignment structures provide positioning information useful for aligning the framed membrane with the manufacturing device or machine (e.g., platen, catalyst spraying equipment, diffusion layer mounting apparatus, etc.) so that during the manufacturing process, the manufacturing device or machine is capable of positionally performing an appropriate process on a desired region of the framed membrane. That is, the alignment structures provide a means whereby the position of the framed membrane is “known” relative to the manufacturing device or machine, such that the manufacturing device or machine is capable of directing the appropriate process to the appropriate location on the framed membrane. The alignment structures also optionally provide positioning information for one or more masks, described below, that may be implemented in the MEA manufacturing process. For example, in one aspect, the outer edge of a mask is aligned with the inner edge of the frame. In this manner, the inner edge of the frame acts as an alignment structure for receiving the outer edge (a second alignment structure) of the mask, thereby positioning the mask in a desired position relative to the membrane that is fixed within the frame.
Further still, the framed membrane can, as an assembly, be removably attached to a platen. A platen provides a firm fixture for the framed membrane, and preferably includes alignment structures such as fiducials, guide holes, and/or other indicia that are used by the manufacturing device or machine (e.g., catalyst spraying equipment, diffusion layer mounting apparatus, etc.) to locate the membrane and determine a substantially exact position over it. According to a preferred embodiment of the present invention, the framed membrane is removably secured to the platen is via vacuum means.
Further, the present invention optionally comprises pre-conditioning of the membrane prior to application of the catalyst layers. Additionally or alternatively, the present invention optionally comprises post-conditioning of the MEA, for example, following final assembly. Pre-conditioning provides the benefit of increasing the stability of the membrane, particularly during catalyst ink spraying, and post-conditioning provides the benefit of activating the membrane electrode assemblies prior to insertion into DMFC's, thereby shortening the period of time until they are fully activated and capable of generating power.
According to another embodiment of the present invention, masks are employed in a process for forming a CCM and/or an MEA. Masks may be used, for example, to define an area or region on an electrolyte membrane that is to be sprayed with a sprayable catalyst-containing ink to form one or more CCM's, much like a stencil. In this manner, the masks are used as guides for spraying the sprayable catalyst-containing ink onto an electrolyte membrane. If the process employs multiple sprayable inks, one or more than one mask may be employed as each respective layer is formed from the multiple inks.
Direct Methanol Fuel Cells (DMFC's)
CCM 28 comprises an electrolytic membrane 8 having opposing major planar surfaces and catalyst layers disposed on each of the opposing major planar surfaces, respectively, and which may be formed, for example, from one or more sprayable catalyst-containing inks according to an embodiment of the present invention. Specifically, a first catalyst layer (anode 6) is formed, e.g., through spraying of a first catalyst-containing ink, on a first major planar surface of the electrolytic membrane 8, and a second catalyst layer (cathode 10) is formed, e.g., through spraying of a second catalyst-containing ink, on a second major planar surface of the electrolytic membrane 8.
As shown in
General Flow Diagram of a Method for Manufacturing MEA's
The method for manufacturing MEA's for use in DMFC's according to an embodiment of the present invention comprises four major steps. In step 102, membrane 8 is prepared for the application of cathode layer 10 and anode layer 6, though, as one of ordinary skill in the art can appreciate, the layers do not necessarily have to be applied in that particular order. Membrane 8, as is well known to those of ordinary skill in the art of the present invention, comprises an electrolytic material, which, in a preferred embodiment, comprises a sulfonated tetrafluoroethylene copolymer such as NAFION®. The anode and cathode layers 6, 10 are formed on the opposing major planar surfaces of the membrane using a sprayable catalyst-containing ink according to an embodiment of the present invention. The sprayable catalyst-containing ink, or fluid, comprises anodic or cathodic catalyst particles (depending on which layer that is being applied), a vehicle (e.g., water), and preferably electrolyte, e.g., NAFION®, particles. The sprayable catalyst-containing ink, and the method for applying same, according to several embodiments of the present invention, are discussed in greater detail below. After application of the anode catalyst and cathode catalyst, the assembly is referred to as a catalyst coated membrane (CCM) 28. Step 102 of method 100 illustrated in
Following the preparation of membrane 8, and the application of the anode and cathode layers in step 102, the diffusion layers 18, 16 are prepared and applied to the CCM 28 in steps 104, 106 respectively. According to a preferred embodiment of the present invention, the fuel cell is a direct methanol fuel cell (DMFC) 20, wherein a methanol-containing fuel is applied to the anode in a liquid state, as shown in
In the sequence shown in
Following step 108, in which post-conditioning occurs to CCM's 28, post-processing and packing step 110 may be implemented. Following post-processesing and packaging, the MEA's 29 may be ready for shipping. Step 110 of method 100 illustrated in
Detailed Flow Diagram for Manufacturing MEA's
According to a preferred embodiment of the present invention, membrane 8 is fabricated from and hence comprises a sulfonated tetrafluoroethylene copolymer such as NAFION®. According to a preferred embodiment of the present invention, membrane 8 is pre-conditioned by bathing membrane 8 in a bath comprising water at an elevated temperature and an acid, or water (without acid) at an elevated temperature, or an acid. Preferably, membrane 8 is pre-conditioned with a protonating agent. A protonating agent may be regarded as any type of acidic solution. The process of pre-conditioning improves the stability of membrane 8 and substantially inhibits wrinkling from occurring during or after subsequent spraying steps, described below. In optional step 304, conditioned membrane 8 is stored. According to a preferred embodiment of the present invention, membrane 8 is stored wet in a substantially pure deionized (DI) water storage container. Shown in
With continued reference to
According to a preferred embodiment of the present invention, frame 60, which is preferably a substantially rigid structure, comprises one or more alignment structures configured to orient frame 60 and membrane 8 in a reference position relative to one or more devices or machines, e.g., automated apparatuses, configured to perform one or more operations on framed membrane 8. According to a preferred embodiment of the present invention, the alignment structure comprises a pin or peg, a pin or peg hole (e.g., for receiving an alignment pin or peg), an optical alignment device, or a frame edge. The alignment structures assist automated manufacturing equipment in precisely locating the frame and membrane 8 such that precise application of sprayed catalyst materials can occur, as well as many different other operations. Preferably, the automated manufacturing equipment comprises software driven-computer controlled automated manufacturing equipment (which optionally includes “pick-and-place” equipment) that utilizes the alignment structures to repeatably and precisely perform various operations on membrane 8 during the MEA manufacturing process. Another benefit of the software-driven-computer controlled manufacturing equipment is that changes to the designs of MEA's can literally be made “on-the-fly”, from one MEA to another, with substantially little or no “down time” in the manufacturing process, other than re-loading of different supplies (if necessary) and minimal software design modification.
As discussed above, various operations can be performed by the software-driven-computer controlled manufacturing equipment. According to a preferred embodiment of the present invention, these operations may comprise, for example, one or more of the following: spraying of one or a plurality of catalyst layers on the framed membrane 8, placing one or more diffusion layers on a catalyst coated membrane 8, laminating a catalyst coated membrane 8 with one or more diffusion layers, marking or labeling assembled membrane electrode assemblies, cutting one or more assembled membrane electrode assemblies, and placing gaskets on the one or more assembled membrane electrode assemblies. These steps are described in greater detail below.
According to an embodiment of the present invention, frame 60 comprises an inner edge 61 (see
Platen 64, shown in
Reverting to
In optional step 312, identification ink is retrieved, and in optional step 314, membrane 8 or frame 60 is labeled using the identification ink. The label can comprise a unique identification number and/or barcode that can be applied via an inkjet or other printing process.
At this point in the manufacturing process, membrane 8 is ready for formation of the cathode and anode catalyst layers. According to an exemplary embodiment of the present invention, membrane 8 has preferably been pre-conditioned with either a hot-water and acid bath, a hot water bath, or just an acid bath, or according to still another method, via hydrogen peroxide, sulfuric acid and distilled water baths, and has been positioned on frame 60 that is preferably removably secured to platen 64 by a vacuum. Platen 64 provides a firm fixture for membrane 8, and preferably includes fiducials, pins, pegs, guide holes, and/or other indicia that may be used by the catalyst spraying equipment (as well as other equipment used to form the MEA) to locate membrane 8 and determine a substantially exact position over it. Therefore, the anode and cathode layers 6, 8 can be applied onto membrane 8 substantially precisely, such as to within, according to an exemplary embodiment of the present invention, ±about 1 mm, ±about 0.5 mm or ±about 0.2 mm.
In step 318, cathode mask and anode mask are designed and prepared. Masks can be prepared using a variety of machining techniques, e.g., water cutting, laser cutting, and other standard machining techniques. Cathode mask and anode mask can be formed from a wide variety of materials, such as, for example, stainless steels, low VOC plastics, or aluminums. In some fuel cell designs, anode mask will be the same as or a mirror image of cathode mask, and in other designs, anode mask will be different. In some embodiments, multiple cathode masks and/or multiple anode masks may be employed, for example, to form a cathode catalyst layer and/or an anode catalyst layer having a catalyst gradient or an electrolyte (e.g., NAFION®) gradient (i.e., in the x, y and/or z directions).
In step 316, a mask is applied to a first surface of framed membrane 8, and in step 320, a mask (same or different from the mask used in step 316) is applied to a second surface of framed membrane 8. Prior to each of steps 316 and 320, platen 64 and framed membrane 8, which is removably secured thereto, are preferably heated to a temperature of from about 50° C. to about 100° C., e.g., from about 60° C. to about 80° C., preferably about 70° C. Heating of platen 64 and membrane 8 prior to and during spraying of the catalyst-containing ink(s) desirably facilitates vehicle removal and formation of highly porous catalyst layers on the first and second surfaces of membrane 8.
The purpose of cathode mask is to allow a sprayable cathode catalyst-containing ink to be deposited onto a first surface of a membrane in a first area (or pattern) and to substantially prevent the sprayable cathode catalyst-containing ink from being deposited in a second area. Similarly, the purpose of anode mask is to allow a sprayable anode catalyst-containing ink to be deposited onto a second surface of the membrane in a third area (or pattern) and to substantially prevent the sprayable anode catalyst-containing ink from being deposited in a fourth area. Optionally, the first area is substantially the same pattern as the third area, and the second area is substantially the same pattern as the fourth pattern. In another aspect, the first area is the negative or inverse of the third area, and the second area is the negative or inverse of the fourth pattern. In still another aspect, the pattern of the first area is unrelated to the pattern of the third area, and the pattern of the second area is unrelated to the pattern of the fourth area. As discussed below, masks can be very simple in design (e.g., a single large open area, with a border portion), or can have nearly any imaginable design to create, for example, localized gradients of catalyst material as desired.
In another embodiment, for example where multiple CCM's are formed in a batch process, the mask resembles the diffusion layer alignment jig 96 shown in
A further non-limiting example of a mask 48b is shown in
Another possible mask 48c is shown in
The results of spraying catalyst-containing ink 44 onto a membrane 8 with masks 48a, 48b, 48c, are shown in
Shown in all three masks 48a, 48b and 48c is a fiducial 88. Fiducial 88 is an example of an alignment structure, in this case a mark, that can be used by automated manufacturing equipment to determine a known location or reference point, so that other operations can occur based on known distances/locations from that fiducial. Alternatively, fiducial 88 can be an alignment opening for receiving alignment pins located on frame 60 (or, conversely, alignment pins for interacting with an alignment opening). According to a preferred embodiment of the invention, there would generally be at least two such alignment pins, though that is not always necessarily the case. As discussed above, mask 48 has an outer edge 49 that can be used to align the masks with frame 60, using frame inner edge 61 (See
Reverting to
The spraying pattern illustrated in
Spraying nozzle 34 sprays anode or cathode catalyst-containing ink that is fed to it in a finely controlled aerosol or mist spray. Membrane 8 is held, as discussed above, onto platen 64, which is preferably heated to a temperature, e.g., a temperature of from about 50 to about 80° C. As the catalyst-containing ink is forcibly ejected from nozzle 34, vehicle 42 substantially, partially or wholly evaporates, and, upon contacting the membrane 8 (or a previously applied catalyst layer), the catalyst particles 40 and electrolyte particles 38 adhere to the surface of membrane 8 as agglomerates 46.
According to an exemplary embodiment of the present invention, a sonicasting/recirculating system is used to improve the uniformity (dispersion) of the catalyst-containing ink and break-up any agglomerations in the catalyst-containing ink prior to spraying.
As discussed above, steps 104 and 106 of process 100 shown in
The preparation and application of gas diffusion layer 18 and liquid diffusion layer 16 are substantially similar. Therefore, for the purpose of brevity, the preparation and application of the gas diffusion layer 18 and liquid diffusion layer 16 shall be discussed below with reference generically to the application of “diffusion layers.”
Following step 326, the assembly is referred to as a catalyst coated membrane (CCM) 28. CCM's 28 can be packaged and shipped to customers who want to apply their own diffusion layers. Alternatively, the CCM's may be manufactured into MEA's and packaged and shipped as MEA's. Thus, step 328 and 340, preparation (e.g., conditioning) of the diffusion layers occurs, if necessary. The material used to make diffusion layers can be obtained and pre-conditioned, if necessary or desired. Therefore, steps 328 and 340 are optional. The diffusion layers may be manufactured, for example, from either a cloth material impregnated with carbon (referred to as carbon-cloth, or C-cloth) or a piece of treated or untreated paper.
In steps 330 and 342 of
In steps 334 and 346, an adhesive material is applied to the cut out diffusion layer pieces. Application of the adhesive material can be done, for example, in two different ways. According to a preferred embodiment, thermally setting adhesive is applied to a perimeter of the diffusion layer material. Then, in steps 336 and 348, the diffusion material with thermal adhesive is applied to CCM 28 on its cathode and anode sides, respectively. Once applied to the CCM, the gas diffusion layer material is referred to as a gas diffusion layer (GDL) 18, and the liquid diffusion layer material is referred to as a liquid diffusion layer (LDL) 16. According to another embodiment of the present invention, an electrolyte-containing ink (e.g., comprising NAFION® as the electrolyte) may be sprayed substantially over one entire side of the diffusion layer material prior to its placement on the cathode side of membrane 8. In this aspect, the electrolyte in the electrolyte-containing ink exhibits glue-like properties and secures the diffusion layer to the membrane. The spraying may occur with the same or different spraying nozzle used to apply the catalyst-containing ink to the membrane during CCM manufacturing.
In step 338 of method 300, frame assembly 60 is lifted, flipped, and aligned again with platen 64 such that the anode side of membrane 8 is facing up. Then, steps 340, 342, 344, 346 and 348 occur (shown in
The process described above can be automated, or performed manually. According to a preferred embodiment of the present invention, the process of applying the diffusion layer materials is performed automatically, with robots and other computer controlled machines performing substantially all of the process steps. In that circumstance, method 300 can optionally prepare one or a plurality of MEA's 29 at a time. In the latter instance, efficiencies of scale are realized, as is well known to those familiar with the art of manufacturing processes. In this latter case, certain different options may also occur, and these will now be discussed in greater detail.
As discussed above, in steps 336 and 348, gas diffusion layer material (GDL) 17 and liquid diffusion layer material (LDL) 15 was first coated (either selectively, or wholly) with an adhesive material, and then applied to a cathode and anode side, respectively. In the case of a large scale automated manufacturing process, wherein multiple MEA's 29 will be manufactured at one time, this process (as well as steps 330, 332, 334, 342, 344, and 346) may be slightly modified. Instead of only one piece of diffusion layer, multiple pieces can be treated at once.
According to an alternative embodiment of the present invention (not shown), the mask can be removed prior to insertion of the alignment jig 96 and then the alignment jig 96 fits within frame 60. Whether the alignment jig 96 fits within frame 60 or the mask depends on the design of the MEA. According to the embodiments of the present invention illustrated in
In
Method 300 illustrates and describes the process for manufacturing CCM 28 and subsequently manufacturing or forming MEA 29. In the aforementioned process, as shown in steps 328-336, and 340-348, the GDLM 17 and then LDLM 17 are secured to their respective sides of CCM 28, and then, as described above, secured in a single pressing step 350. According to an alternative embodiment of the present invention, multiple pressing steps may be employed, e.g., one for each side of the CCM. In this process, for example, an alignment jig can be aligned with the first major planar surface (i.e., the cathode side of CCM 28), and multiple GDLM's 17 are inserted into the alignment jig 96 openings 98. Then, MEA press 102 is used to press GDLM's 17 onto CCM 28. The alignment jig 96 is then removed, and optionally, the CCM (having the first diffusion layer secured to it) is flipped over. The process can then be repeated for the anode side of the CCM, using the same or different alignment jig 96. LDLM's 15 are inserting into the openings in the same or different alignment jig and placed onto the second major planar surface of CCM 28 (i.e., the anode catalyst layer). A second pressing operation is then performed on inserted LDLM's 15 using press 102. Following the second pressing step, the alignment jig 96 is removed. A single alignment jig optionally may be used for both the first and second pressing operations, or different alignment jigs may be used.
Following step 350 (which is optionally repeated from both anode and cathode sides of CCM 28), MEA 29 is formed. Then, in optional step 352 post-conditioning occurs. According to a preferred embodiment of the present invention, post-conditioning comprises bathing the CCM (i.e., within the formed MEA) at an elevated temperature in a first bath comprising either or both water and acid, optionally at an elevated temperature. Post-conditioning preferably removes potential contaminates left from the ink vehicle or process handling. According to an alternative embodiment of the present invention, post-conditioning also comprises alternating baths of distilled water and sulfuric acid.
In step 354, each MEA is cut with a laser to ensure accurate dimensions, and then retrieved and placed into another tray. In step 356, each MEA is inspected, or randomly selected according to established inspection quality standards (i.e., six-sigma (6a) ISO 9000 quality standards) and in step 358, the inspected MEA's are packaged. In step 360, the packaged MEA's 29 are shipped.
According to a preferred embodiment of the present invention, the process for forming a plurality of MEA's 29 is performed by software-driven, fully automated manufacturing devices, including, for example, pick-and-place robots. According to an alternative embodiment of the invention, the method for forming a plurality of MEA's can be a combination of manual and automated processes, wherein the automated processes, in either the fully or partial automated process, comprise the steps of inserting the plurality of first and second diffusion layer materials (i.e., GDLM 17, LDLM 15) into openings 98 on alignment jig 96.
Any of the steps of the processes of the present invention and those that follow that involve the movement of materials, the application of inks, and in general the entire process, can be performed manually, or automatically, or some combination of the two, even on a step-by-step process. For example, in step 322 discussed above, the application of cathode catalyst-containing ink 44 can be accomplished by a technician using a spray gun, or it can be accomplished by a robot-machine that follows instructions pre-programmed into it to spray the fluid on the membrane surface. Additionally, the steps may be performed in any order so long as the order is logically possible.
The present invention has been described with reference to certain exemplary embodiments thereof. However, it will be readily apparent to those skilled in the art that it is possible to embody the invention in specific forms other than those of the exemplary embodiments described above. This may be done without departing from the spirit and scope of the invention. The exemplary embodiments are merely illustrative and should not be considered restrictive in any way. The scope of the invention is defined by the appended claims and their equivalents, rather than by the preceding description.
All United States patents and applications, foreign patents, and publications discussed above are hereby incorporated herein by reference in their entireties.
